Today, a plurality of information processing apparatuses such as computers are connected to a network, and data communication is performed between apparatuses via the network. The Transmission Control Protocol/Internet Protocol (TCP/IP) is known as a protocol for packetizing and transmitting data on a network.
In addition, there are cases in which packets flowing on a network are collected and accumulated in a predetermined apparatus and in which the communication quality of the network is monitored on the basis of the collected packets. For example, there are cases in which the round trip time (RTT) of a data communication is measured on the basis of the difference between the arrival time of a data packet and the arrival time of an acknowledgement (ACK) packet in response to the data packet. By using such RTTs, the communication quality can be evaluated or congestion control can be performed.
See, for example, the following documents:
Japanese Laid-open Patent Publication No. 2013-78011
Japanese Laid-open Patent Publication No. 2008-182410
Japanese National Publication of International Patent Application No. 2008-508817
Among such ACK packets, there are delayed ACK packets. When a destination device receives a data packet transmitted from a source device, there are cases in which the destination device transmits a delayed ACK packet. By using such a delayed ACK packet, the destination device can delay transmission of an ACK packet to the source device. For example, the destination device transmits a single delayed ACK packet in response to a plurality of data packets that the destination device has received. In addition, the destination device also transmits an ACK packet if a predetermined period of time elapses before the destination device receives a predetermined number of data packets. Namely, the destination device also transmits an ACK packet if a timeout occurs. In this case, an RTT can still be calculated by using an ACK packet transmitted in response to one data packet among the collected packets. However, since a delayed ACK packet could be transmitted by a timeout, the measurement accuracy of the RTT could be deteriorated. This is because transmission of a delayed ACK packet by a timeout is attributable to the standby time of the destination device and is not related to any delay in the transmission paths on the network.
If a timeout value for such delayed ACK packets is fixed, RTTs can still be calculated by excluding the ACK packets transmitted by the timeout value. However, since the timeout value can vary, it is not easy to exclude the impact caused by a timeout.